Viscometer

ABSTRACT

The viscometer provides a viscosity value (X η ) which represents the viscosity of a fluid flowing in a pipe connected thereto. It comprises a vibratory transducer with at least one flow tube for conducting the fluid, which communicates with the pipe. Driven by an excitation assembly, the flow tube is vibrated so that friction forces are produced in the fluid. The viscometer further includes meter electronics which feed an excitation current (i exc ) into the excitation assembly. By means of the meter electronics, a first internal intermediate value (X 1 ) is formed, which corresponds with the excitation current (i exc ) and thus represents the friction forces acting in the fluid. According to the invention, a second internal intermediate value (X 2 ), representing inhomogeneities in the fluid, is generated in the meter electronics, which then determine the viscosity value (X η ) using the two intermediate values (X 1 , X 2 ). The first internal intermediate value (X 1 ) is preferably normalized by means of an amplitude control signal (y AM ) for the excitation current (i exc ), the amplitude control signal corresponding with the vibrations of the flow tube. As a result, the viscosity value (X η ) provided by the viscometer is highly accurate and robust, particularly independently of the position of installation of the flow tube.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.12/382,489, filed on Mar. 17, 2009, now allowed; which is a divisionalof U.S. application Ser. No. 11/605,260 filed Nov. 29, 2006, nowabandoned; which is a divisional of U.S. application Ser. No. 11/178,431filed on Jul. 12, 2005, now U.S. Pat. No. 7,162,915; which is adivisional of U.S. application Ser. No. 10/835,471 filed on Apr. 30,2004, now U.S. Pat. No. 7,036,355; which is a continuation of Ser. No.10/226,242 filed on Aug. 23, 2002, now U.S. Pat. No. 6,910,366 and whichclaims the benefit of Provisional Application No. 60/322,743, filed Sep.18, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a viscometer for a fluid flowing in a pipe andto a method of determining the viscosity of the fluid.

In measurement and automation technology, the viscosity of a fluidflowing in a pipe, particularly of a liquid, is frequently determined bymeans of meters which, using a vibratory transducer and meterelectronics connected thereto, induce internal friction forces in thefluid and derive therefrom a measurement signal representative of therespective viscosity.

Such viscometers are described, for example, in U.S. Pat. No. 4,524,610or in published International Application WO-A 95/16897 and comprise:

-   a vibratory transducer with an essentially straight flow tube for    conducting the fluid, said flow tube communicating with the pipe and    vibrating in operation,-   an excitation assembly for vibrating the flow tube;-   a central axis of the vibrating flow tube being essentially left in    its shape and spatial position, so that the flow tube practically    does not leave a static rest position assigned to it,-   a sensor arrangement for sensing vibrations of the flow tube and for    generating at least one sensor signal representing the vibrations of    the flow tube; and-   meter electronics which deliver an excitation current for the    excitation assembly and at least one measured value representing the    instantaneous viscosity of the fluid,-   the meter electronics adjusting the excitation current by means of    the at least one sensor signal and-   generating by means of the excitation current an internal    intermediate value representing instantaneous frictions in the    fluid, and-   the meter electronics determining the viscosity value using the    internal intermediate value.

It has turned out, however, that in spite of viscosity and density beingmaintained virtually constant, particularly under laboratory conditions,the viscosity value determined by means of the excitation current mayexhibit considerable inaccuracies, which may amount to as much as onehundred times the actual viscosity of the fluid.

2. Discussion of the Prior Art

In U.S. Pat. No. 4,524,610, a possible cause of this problem isindicated, namely the fact that gas bubbles in the fluid may be trappedat the wall of the flow tube. To avoid this problem, it is proposed toinstall the transducer so that the straight flow tube is in anessentially vertical position, so that the trapping of bubbles isprevented. This, however, is a very specific solution which is onlyconditionally realizable, particularly in industrial process measurementtechnology. On the one hand, the pipe into which the transducer is to beinserted would have to be adapted to the latter and not vice versa,which probably cannot be conveyed to the user. On the other hand, theflow tubes may also have a curved shape, so that the problem cannot besolved by adapting the position of installation. It has also turned outthat the aforementioned inaccuracies of the measured viscosity valuecannot be appreciably reduced even if a vertically installed, straightflow tube is used. Variations in the measured viscosity value of amoving fluid cannot be prevented in this manner, either.

SUMMARY OF THE INVENTION

Therefore, an object of the invention is to provide a viscometer forfluids which, particularly when the fluid is flowing, provides a highlyaccurate and robust viscosity value essentially independently of, on theone hand, the position of installation of the flow tube and, on theother hand, the vibrations of the flow tube, particularly of theiramplitude.

To attain the object, a first variant of the invention provides aviscometer for a fluid flowing in a pipe, said viscometer comprising:

-   a vibratory transducer;-   at least one flow tube for conducting the fluid and for generating    friction forces acting in the fluid, the at least one flow tube    communicating with the pipe and vibrating in operation,-   an excitation assembly for vibrating the at least one flow tube; and-   meter electronics which deliver-   an excitation current for the excitation assembly and-   a viscosity value representing the instantaneous viscosity of the    fluid,-   the meter electronics generating-   a first internal intermediate value, corresponding with the    excitation current and representing the friction forces acting in    the fluid, and-   a second internal intermediate value, representing inhomogeneities    in the fluid, and-   the meter electronics determining the viscosity value using the    first and second internal intermediate values.

A second variant of the invention provides a viscometer for a fluidflowing in a pipe, said viscometer comprising:

-   a transducer, particularly a flexural mode transducer,-   at least one flow tube for conducting the fluid and for producing    friction forces acting in the fluid, the at least one flow tube    communicating with the pipe and vibrating in operation,-   an excitation assembly for vibrating the at least one flow tube, and-   a sensor arrangement for sensing vibrations of the flow tube and for    generating at least a first sensor signal representing said    vibrations; and-   meter electronics which deliver-   an excitation current for the excitation assembly and-   a viscosity value representing the instantaneous viscosity of the    fluid,-   the meter electronics-   deriving from the at least first sensor signal an amplitude control    signal serving to adjust the excitation current, and-   determining the viscosity value by means of the at least first    sensor signal and the amplitude control signal.

Furthermore, the invention provides a method of determining theviscosity of a fluid flowing in a pipe, said method comprising the stepsof:

-   -   feeding an excitation current into an excitation assembly        mechanically coupled to a flow tube conducting the fluid, for        causing mechanical vibrations, particularly flexural vibrations,        of the flow tube;    -   vibrating the flow tube for producing internal friction forces        in the fluid;    -   sensing vibrations of the flow tube for generating a first        internal intermediate value, representing friction forces acting        in the fluid;    -   producing a sampling of first internal intermediate values;    -   using the sampling to determine a second internal intermediate        value, representing inhomogeneities in the fluid; and    -   generating a measured viscosity value by means of the two        internal intermediate values.

In a first preferred embodiment of the viscometer of the invention, themeter electronics determine the second internal intermediate value bymeans of the excitation current.

In a second preferred embodiment of the viscometer of the invention, theflow tube for producing friction forces acting in the fluid vibrates atleast in part in a flexural mode.

In a third preferred embodiment of the viscometer of the invention, togenerate the measured viscosity value, the meter electronics determineby means of the at least first sensor signal a velocity value whichrepresents motions causing the friction forces in the fluid.

In a fourth preferred embodiment of the viscometer of the invention, themeter electronics determine the first internal intermediate value, whichcorresponds with the excitation current and represents the frictionforces acting in the fluid, by means of the amplitude control signal.

In a fifth preferred embodiment of the viscometer of the invention, togenerate the viscosity value, the meter electronics normalize the firstinternal intermediate value to the velocity value.

In a sixth preferred embodiment of the viscometer of the invention, themeter electronics comprise a volatile data memory holding a sampling offirst internal intermediate values, and generate the second internalintermediate value by means of the sampling.

In a seventh preferred embodiment of the viscometer of the invention,the second internal intermediate value is generated using a standarddeviation of the first internal intermediate value, which is estimatedby means of the sampling.

In a first preferred embodiment of the method of the invention, at leastone sensor signal representing the vibrations of the flow tube isgenerated, and the excitation current is adjusted using the at least onesensor signal.

In a second preferred embodiment of the method of the invention, avelocity value which represents a velocity of a motion causing thefriction forces acting in the fluid is determined using the sensorsignal, and the first internal intermediate value is normalized to thevelocity value.

The invention is predicated on recognition that the excitation power fedinto the transducer to maintain the vibrations of the flow tube, andthus the viscosity value derived therefrom, may be affected in adisproportionately great measure by inhomogeneities in the fluid, suchas gas bubbles or entrained particles of solid matter. The invention isalso predicated on recognition that the excitation power can also bedetermined very precisely in a very simple manner by means of adjustmentsignals or adjustment values generated within the meter electronics, andthat both the adjustment signals for the excitation power and theactually injected excitation power, particularly in the case offlow-tube vibrations with an amplitude regulated at constant value, aretoo inaccurate by themselves for robust viscosity measurement.

A fundamental idea of the invention is to derive from the excitationpower an internal measured value, particularly independently of the typeof fluid involved, which represents the inhomogeneities in the fluidrelevant to the viscosity measurement and which is a measure of theireffect on the measured viscosity value.

Another fundamental idea of the invention is to determine the viscosityvalue by means of adjustment signals or adjustment values for theexcitation power that are generated within the meter electronics, and bymeans of vibrations of the flow tube that are maintained by the actuallyinjected excitation power. This indirect determination of the excitationpower has the advantage of eliminating the need for an additionalmeasurement of the injected excitation power for the purpose ofdetermining the viscosity value.

A further advantage of the invention therefore consists in the fact thatit can also be implemented in commercially available Coriolis massflowmeter-densimeters, for example, and virtually independently of theconcrete shape and number of flow tubes used in the respectivetransducer, i.e., without any basic change in the mechanical design ofthe transducer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further advantages will become more apparent from thefollowing description of embodiments taken in conjunction with theaccompanying drawings, in which like parts are designated by likereference characters throughout the various figures; referencecharacters that have already been allotted are omitted in subsequentfigures if this contributes to clarity. In the drawings:

FIG. 1 is a perspective view of a viscometer for generating a viscosityvalue;

FIG. 2 is a block diagram of a preferred embodiment of meter electronicssuitable for the viscometer of FIG. 1;

FIG. 3 is a part-sectional, first perspective view of an embodiment of avibratory transducer suitable for the viscometer of FIG. 1;

FIG. 4 is a second perspective view of the transducer of FIG. 3;

FIG. 5 shows an embodiment of an electromechanical excitation assemblyfor the transducer of FIG. 3; and

FIG. 6 is a diagram symbolizing steps implemented in the meterelectronics for the determination of the viscosity value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 and 2 show schematically a viscometer with a vibratorytransducer 10, preferably housed in a transducer case 100, and withmeter electronics 50, housed in an electronics case 200 and, as shown inFIG. 2, electrically connected to transducer 10. The viscometer servesin particular to sense a viscosity, η, of a fluid flowing in a pipe (notshown) and to map this viscosity into a measured viscosity value X_(η)representing this viscosity η. By means of transducer 10, which isdriven by meter electronics 50, friction forces are generated in thefluid passing therethrough which are dependent on the viscosity η andreact on transducer 10 in a measurable manner, i.e., which can bedetected using sensor technology and converted into useful input signalsfor subsequent evaluation electronics.

In the preferred case where the viscometer is designed to be coupled toa field bus, the, preferably programmable, meter electronics 50 willinclude a suitable communication interface for data communication, e.g.,for the transmission of the measurement data to a higher-level storedprogram control or a higher-level process control system.

FIGS. 3 and 4 show an embodiment of a transducer 10 in the form of aphysical-to-electrical vibratory transducer assembly. The constructionof such a transducer assembly is described in detail in U.S. Pat. No.6,006,609, for example. Such transducers are already used incommercially available Coriolis mass flowmeter-densimeters as areoffered by the applicant with its “PROMASS I” series, for example.

To conduct the fluid to be measured, transducer 10 comprises at leastone flow tube 13 of a predeterminable, elastically deformable lumen 13Aand a predeterminable nominal diameter, which has an inlet end 11 and anoutlet end 12. “Elastic deformation” of lumen 13A as used herein meansthat in order to produce reaction forces in the fluid, i.e., forcesdescribing the fluid, namely shearing or friction forces, but alsoCoriolis forces and/or mass inertial forces, in operation, athree-dimensional shape and/or a spatial position of lumen 13A arechanged in a predeterminable cyclic manner, particularly periodically,within an elasticity range of flow tube 13; see, for example, U.S. Pat.No. 4,801,897, U.S. Pat. No. 5,648,616, U.S. Pat. No. 5,796,011, and/orU.S. Pat. No. 6,006,609.

At this point it should be noted that instead of a transducer accordingto the embodiment of FIGS. 3 and 4, virtually any transducer known tothe person skilled in the art for Coriolis flowmeter-densimeters,particularly a flexural mode transducer with a bent or straight flowtube vibrating exclusively or at least in part in a flexural mode, canbe used for implementing the invention. Further suitable implementationsof transducer assemblies that can be used for transducer 10 aredescribed, for example, in U.S. Pat. Nos. 5,301,557, 5,357,811,5,557,973, 5,602,345, 5,648,616, or 5,796,011, which are incorporatedherein by reference.

Materials especially suited for flow tube 13, here an essentiallystraight tube, are titanium alloys, for example. Instead of titaniumalloys, other materials commonly used for such flow tubes, particularlyfor bent tubes, such as stainless steel or zirconium, may be employed.

Flow tube 13, which communicates at the inlet and outlet ends with thefluid-conducting pipe in the usual manner, is clamped in a rigid supportframe 14, particularly in a flexurally and torsionally stiff frame, soas to be capable of vibratory motion, the support frame being preferablyenclosed by a transducer case 100.

Support frame 14 is fixed to flow tube 13 by means of an inlet plate 223at the inlet end and by means of an outlet plate 213 at the outlet end,the two plates being penetrated by respective corresponding extensionpieces of flow tube 13. Support frame 14 has a first side plate 24 and asecond side plate 34, which are fixed to inlet plate 213 and outletplate 223 in such a way as to extend essentially parallel to and inspaced relationship from flow tube 13; see FIG. 3. Thus, facing sidesurfaces of the two side plates 24, 34 are also parallel to each other.

Advantageously, a longitudinal bar 25 serving as a balancing mass forabsorbing vibrations of flow tube 13 is secured to side plates 24, 34 inspaced relationship from flow tube 13. As shown in FIG. 4, longitudinalbar 25 extends essentially parallel to the entire oscillable length offlow tube 13; this, however is not mandatory; if necessary, longitudinalbar 25 may also be shorter, of course.

Thus, support frame 14 with the two side plates 24, 34, inlet plate 213,outlet plate 223, and the optional longitudinal bar 25 has alongitudinal axis of gravity which is essentially parallel to a centralflow tube axis 13B, which joins inlet end 11 and outlet end 12.

In FIGS. 3 and 4, it is indicated by the heads of the screws shown thatthe aforementioned fixing of side plates 24, 34 to inlet plate 213, tooutlet plate 223 and to longitudinal bar 25 may be done by screwing; itis also possible to use other suitable forms of fastening familiar tothose skilled in the art.

If transducer 10 is to be nonpermanently connected with the pipe, flowtube 13 preferably has an inlet-side first flange 19 and an outlet-sidesecond flange 20 formed thereon, see FIG. 1; instead of flanges 19, 20,so-called Triclamp connections, for example, may be used to provide thenonpermanent connection with the pipe, as indicated in FIG. 3.

If necessary, however, flow tube 13 may also be connected with the pipedirectly, e.g., by welding or brazing.

To produce the above-mentioned friction forces, during operation oftransducer 10, flow tube 13, driven by an electromechanical excitationassembly 16 coupled to the flow tube, is caused to vibrate in theso-called useful mode at a predeterminable frequency, particularly at anatural resonance frequency which is also dependent on a density, ρ, ofthe fluid, whereby the flow tube is elastically deformed in apredeterminable manner.

In the embodiment shown, the vibrating flow tube 13, as is usual withsuch flexural mode transducer assemblies, is spatially, particularlylaterally, deflected from a static rest position; the same applies totransducer assemblies in which one or more bent flow tubes performcantilever vibrations about a corresponding longitudinal axis joiningthe respective inlet and outlet ends, or to those in which one or morestraight flow tubes perform only planar flexural vibrations about theirlongitudinal axis. In the other case where transducer 10 is a radialmode transducer assembly and the vibrating flow tube is symmetricallydeformed in the usual manner as is described, for example, in WO-A95/16897, the flow tube is essentially left in its static rest position.

Excitation assembly 16 serves to produce an excitation force F_(exc)acting on flow tube 13 by converting an electric excitation powerP_(exc) supplied from meter electronics 50. The excitation power P_(exc)serves virtually only to compensate the power component lost in thevibrating system because of mechanical and fluid friction. To achieve ashigh an efficiency as possible, the excitation power P_(exc) ispreferably precisely adjusted so that essentially the vibrations of flowtube 13 in the useful mode, e.g., those at a lowest resonance frequency,are maintained.

For the purpose of transmitting the excitation force F_(exc) to flowtube 13, excitation assembly 16, as shown in FIG. 5, has a rigid,electromagnetically and/or electrodynamically driven lever arrangement15 with a cantilever 154 and a yoke 163, the cantilever 154 beingrigidly fixed to flow tube 13. Yoke 163 is rigidly fixed to an end ofcantilever 154 remote from flow tube 13, such that it lies above andextends transversely of flow tube 13. Cantilever 154 may be a metallicdisk, for example, which receives flow tube 13 in a bore. For furthersuitable implementations of lever arrangement 15, reference is made tothe above-mentioned U.S. Pat. No. 6,006,609. As is readily apparent fromFIGS. 3 and 5, lever arrangement 15, here a T-shaped arrangement, ispreferably arranged to act on flow tube 13 approximately midway betweeninlet end 11 and outlet end 12, so that in operation, flow tube 13 willexhibit its maximum lateral deflection in the middle.

To drive the lever arrangement 15, excitation assembly 16, as shown inFIG. 5, comprises a first excitation coil 26 and an associated firstarmature 27 of permanent-magnet material as well as a second excitationcoil 36 and an associated second armature 37 of permanent-magnetmaterial. The two excitation coils 26 and 36, which are preferablyelectrically connected in series, are fixed to support frame 14 on bothsides of flow tube 13 below yoke 163, particularly nonpermanently, so asto interact in operation with their associated armatures 27 and 37,respectively. If necessary, the two excitation coils 26, 36 may, ofcourse, be connected in parallel.

As shown in FIGS. 3 and 5, the two armatures 27, 37 are fixed to yoke163 at such a distance from each other that during operation oftransducer 10, armature 27 will be penetrated essentially by a magneticfield of excitation coil 26, while armature 37 will be penetratedessentially by a magnetic field of excitation coil 36, so that the twoarmatures will be moved by the action of corresponding electrodynamicand/or electromagnetic forces.

The motions of armatures 27, 37 produced by the magnetic fields ofexcitation coils 26, 36 are transmitted by yoke 163 and cantilever 154to flow tube 13. These motions of armatures 27, 37 are such that yoke163 is displaced from its rest position alternately in the direction ofside plate 24 and in the direction of side plate 34. A correspondingaxis of rotation of lever arrangement 15, which is parallel to theabove-mentioned central axis 13B of flow tube 13, may pass throughcantilever 154, for example.

Particularly in order to hold excitation coils 26, 36 and individualcomponents of a magnetic brake assembly 217, which is described below,support frame 14 further comprises a holder 29 for electromechanicalexcitation assembly 16. Holder 29 is connected, preferablynonpermanently, with side plates 24, 34.

In the transducer 10 of the embodiment, the lateral deflections of thevibrating flow tube 13, which is firmly clamped at inlet end 11 andoutlet end 12, simultaneously cause an elastic deformation of its lumen13A; this elastic deformation extends virtually over the entire lengthof flow tube 13.

Furthermore, due to a torque acting on flow tube 13 via leverarrangement 15, torsion is induced in flow tube 13 about central axis13B simultaneously with the lateral deflections, at least in sections ofthe tube, so that the latter vibrates in a mixed flexural and torsionalmode serving as a useful mode. The torsion of flow tube 13 may be suchthat the direction of a lateral displacement of the end of cantilever154 remote from flow tube 13 is either the same as or opposite to thatof the lateral deflection of flow tube 13. In other words, flow tube 13can perform torsional vibrations in a first flexural and torsional mode,corresponding to the former case, or in a second flexural and torsionalmode, corresponding to the latter case. In the transducer 10 accordingto the embodiment, the natural resonance frequency of the secondflexural and torsional mode, e.g., 900 Hz, is approximately twice ashigh as that of the first flexural and torsional mode.

In the preferred case where flow tube 13 is to perform vibrations onlyin the second flexural and torsional mode, excitation assemblyadvantageously incorporates a magnetic brake assembly 217 based on theeddy-current principle, which serves to stabilize the position of theaxis of rotation. By means of magnetic brake assembly 217 it can thus beensured that flow tube 13 always vibrates in the second flexural andtorsional mode, so that any external interfering effects on flow tube 13will not result in a spontaneous change to another flexural andtorsional mode, particularly to the first. Details of such a magneticbrake assembly are described in detail in U.S. Pat. No. 6,006,609, forexample; furthermore, the use of such magnetic brake assemblies is knownfrom transducers of the aforementioned “PROMASS I” series.

At this point it should be mentioned that in the flow tube 13 deflectedin this manner according to the second flexural and torsional mode, thecentral axis 13B is slightly deformed, so that during the vibrations,this axis spreads a slightly curved surface rather than a plane.Furthermore, a path curve lying in this surface and described by themidpoint of the central axis of the flow tube has the smallest curvatureof all path curves described by this central axis.

To detect the deformations of flow tube 13, transducer 10 comprises asensor arrangement 60 with at least a first sensor 17, which provides afirst, preferably analog, sensor signal s₁ in response to vibrations offlow tube 13. As is usual with such transducers, sensor 17 may beformed, for example, by an armature of permanent-magnet material fixedto flow tube 13 and interacting with a sensor coil held by support frame14.

Sensor types especially suited for sensor 17 are those which sense thevelocity of the deflections of the flow tube based on the electrodynamicprinciple. It is also possible to use acceleration-measuringelectrodynamic or displacement-measuring resistive or optical sensors,or other sensors familiar to those skilled in the art which are suitablefor detecting such vibrations.

In a preferred embodiment of the invention, sensor arrangement 60further comprises a second sensor 18, particularly a sensor identical tothe first sensor 17, which second sensor 18 provides a second sensorsignal s₂ representing vibrations of the flow tube. In this embodiment,the two sensors 17, 18 are positioned at a given distance from eachother along flow tube 13, particularly at the same distance from themiddle of flow tube 13, such that sensor arrangement 60 detects bothinlet-side and outlet-side vibrations of flow tube 13 and providescorresponding sensor signals s₁ and s₂, respectively. The first sensorsignal s₁ and, if present, the second sensor signal s₂, which usuallyeach have a frequency corresponding to the instantaneous vibrationfrequency of flow tube 13, are fed to meter electronics 50, as shown inFIG. 2.

To vibrate the flow tube 13, excitation assembly 16 is supplied frommeter electronics 50 with a likewise oscillating, unipolar or bipolarexcitation current i_(exc) of adjustable amplitude and adjustablefrequency f_(exc), such that in operation, excitation coils 26, 36 aretraversed by this current to produce the magnetic field necessary tomove armatures 27, 37. Thus, the excitation force F_(exc) required tovibrate flow tube 13 can be monitored and adjusted in amplitude, e.g.,by means of a current- and/or voltage-regulator circuit, and infrequency, e.g., by means of a phase-locked loop, in the manner familiarto those skilled in the art. The excitation current i_(exc) delivered bymeter electronics 50 is preferably a sinusoidal current, but it may alsobe a pulsating, triangular, or square-wave alternating current, forexample.

As is usual in viscometers of the kind being described, the frequencyf_(exc) of the excitation current i_(exc) is equal to the predeterminedvibration frequency of flow tube 13, and is therefore preferably set atan instantaneous natural resonance frequency of the fluid-carrying flowtube 13. As indicated above, the invention proposes for the transducer10 according to the embodiment that the excitation current i_(exc)should be caused to flow through the two excitation coils 26, 36 andthat its frequency f_(exc) should be chosen so that the laterallyoscillating flow tube 13 is, if possible, twisted exclusively accordingto the second flexural and torsional mode.

To generate and adjust the excitation current i_(exc), meter electronics50 comprise a driver circuit 53 which is controlled by a frequencycontrol signal y_(FM), representing the excitation frequency to beadjusted, f_(exc), and by an amplitude control signal y_(AM),representing the amplitude of excitation current i_(exc) to be adjusted.The driver circuit may be implemented with a voltage-controlledoscillator followed by a voltage-to-current converter, for example;instead of an analog oscillator, a numerically controlled digitaloscillator, for example, may be used to adjust the excitation currenti_(exc).

The amplitude control signal y_(AM) may be generated with an amplitudecontrol circuit 51 incorporated in meter electronics 50, which updatesat least one of the two sensor signals s₁, s₂ and the amplitude controlsignal y_(AM) based on the instantaneous amplitude and on a constant orvariable amplitude reference value W₁, respectively; in addition, aninstantaneous amplitude of the excitation current i_(exc) may be used togenerate the amplitude control signal y_(AM). Such amplitude controlcircuits are familiar to those skilled in the art. As an example of suchan amplitude control circuit, reference is again made to Coriolis massflowmeters of the “PROMASS I” series. Their amplitude control circuit ispreferably designed so that the lateral vibrations of flow tube 13 aremaintained at a constant amplitude, i.e., at an amplitude which is alsoindependent of the density ρ.

The frequency control signal y_(FM) may be provided by a suitablefrequency control circuit 52 which updates this signal based, forexample, on at least the sensor signal s₁ and on a DC voltage that isrepresentative of the frequency to be adjusted and serves as a frequencyreference value W₂.

Preferably, frequency control circuit 52 and driver circuit 53 areinterconnected to form a phase-locked loop which is used in the mannerfamiliar to those skilled in the art to keep the frequency controlsignal y_(FM) in phase with an instantaneous resonance frequency of flowtube 13 based on a phase difference measured between at least one of thesensor signals s₁, s₂ and the excitation current to be adjusted or themeasured excitation current, i_(exc). The configuration and use of suchphase-locked loops for driving flow tubes at one of their mechanicalresonance frequencies are described in detail U.S. Pat. No. 4,801,897,for example. Of course, it is also possible to use other frequencycontrol loops familiar to those skilled in the art, such as thosedescribed in U.S. Pat. Nos. 4,524,610 or 4,801,897, for example.Furthermore, regarding the use of such frequency control loops fortransducers of the kind being described, reference is made to theaforementioned “PROMASS I” series.

In a further preferred embodiment of the invention, the amplitudecontrol circuit 51 and the frequency control circuit 52 are implementedby means of a digital signal processor DSP and by means of program codesrunning therein. The program codes may, for instance, be stored in anonvolatile memory EEPROM of a microcomputer 55 controlling and/ormonitoring the signal processor DSP, and be loaded upon start-up ofsignal processor DSP into a volatile data memory RAM of meterelectronics 50, which is incorporated in signal processor DSP, forexample. Signal processors suitable for such applications are, forexample, those of the type TMS320VC33, which are marketed by TexasInstruments Inc.

It goes without saying that for the processing in signal processor DSP,the sensor signal s₁ and, if present, the sensor signal s₂ have to beconverted to corresponding digital signals by means of suitableanalog-to-digital converters A/D; see particularly EP-A 866 319. Ifnecessary, control signals provided by the signal processor, such as theamplitude control signal y_(AM) or the frequency control signal y_(FM),have to be converted from digital to analog form in a correspondingmanner.

Since, as repeatedly indicated, such vibratory transducer assemblies,besides inducing fluid friction forces, also inducemass-flow-rate-dependent Coriolis forces and fluid-density-dependentmass inertial forces, for example, according to a preferred developmentof the invention, the viscometer serves to determine not only theviscosity, η, but also a density, ρ, and a mass flow rate, m, of thefluid, particularly simultaneously, and to provide a correspondingmeasured density value X_(ρ) and a measured mass flow rate value X_(m).This may be done using the methods employed to measure mass flow rateand/or density in conventional Coriolis mass flowmeter-densimeters,particularly in those of the aforementioned “PROMASS I” series, whichmethods are familiar to those skilled in the art; cf. U.S. Pat. Nos.4,187,721, 4,876,879, 5,648,616, 5,687,100, 5,796,011, or 6,073,495.

To generate the measured viscosity value X_(η), meter electronics 50derive from the excitation power P_(exc) fed into excitation assembly16, which power serves in particular to compensate the internal frictionproduced in the fluid in the manner described above, a firstintermediate value X₁, particularly a digital value, which representsthe vibration-damping friction forces in the fluid; in addition to orinstead of the actually injected excitation power P_(exc), an excitationpower predetermined by meter electronics 50 and represented, forexample, by the amplitude control signal y_(AM) and/or the frequencycontrol signal y_(FM) supplied to driver circuit 53, may serve todetermine the viscosity value X_(η) and particularly the intermediatevalue X₁.

In a preferred embodiment of the invention, the intermediate value X₁ isdetermined by means of the excitation current predetermined by meterelectronics 50 and/or by means of the actually injected, measuredexcitation current i_(exc), particularly by means of the amplitude or amoving average of this excitation current. In that case, the excitationcurrent i_(exc) serves as a measure of the entirety of the dampingforces counteracting the deflection motions of the vibrating flow tube13. When using the excitation current i_(exc) to determine theintermediate value X₁, however, the fact that the aforementioned dampingforces are also dependent, on the one hand, on the viscosity-dependentfrictions within the fluid and, on the other hand, on mechanicalfrictions in excitation assembly 16 and in the vibrating flow tube, forexample, has to be taken into account.

To separate the information about the viscosity of the fluid from theexcitation current i_(exc), the latter is therefore reduced in meterelectronics 50 by the value of a no-load current that is virtuallyindependent of the fluid friction, this no-load current being measuredwith flow tube 13 evacuated or at least not filled with liquid. Theusually long-term-stable no-load current can be readily determined inadvance during a calibration of the viscometer, for example, and storedin meter electronics 50, e.g., in the nonvolatile memory EEPROM, in theform of a digital value.

Preferably, the intermediate value X₁ is also formed by simplydetermining a numeric difference between one or more digital excitationcurrent values, representing, for example, the instantaneous amplitudeor an instantaneous average value of the excitation current i_(exc), andthe digital no-load current value. If the excitation current valuerepresents the amplitude or the average value of the excitation currenti_(exc), an amplitude or a corresponding average value of the no-loadcurrent must, of course, be subtracted therefrom to obtain theintermediate value X₁. The excitation current value can be obtained, forexample, by a simple current measurement at the output of driver circuit53. Preferably, however, the excitation current value, and thus theintermediate value X₁, is determined indirectly using the amplitudecontrol signal y_(AM) provided by amplitude control circuit 51, as shownschematically in FIG. 2. This has the advantage of eliminating the needfor additional current measurement and particularly for measuringcircuits necessary therefor.

Taking into account the relationship

√{square root over (η)}˜i_(exc)  (1)

which is described in U.S. Pat. No. 4,524,610 and according to which theexcitation current i_(exc), at least at a constant density ρ, is verywell correlated with the square root of the viscosity, η, in order todetermine of the viscosity value X_(η), first the square of theintermediate value X₁ derived from the excitation current i_(exc) isformed within meter electronics 50.

It turned out that, if the viscosity value X_(η) is determined only bymeans of the intermediate value X₁, it may be much too inaccurate formany industrial applications in spite of the viscosity and densityremaining virtually constant.

Investigations of the phenomenon under laboratory conditions, i.e.,using fluids of known, particularly constant, viscosity and density,have shown that the intermediate value X₁ is highly responsive not onlyto trapped gas bubbles but above all to inhomogeneities in the movingfluid. Such inhomogeneities may be air bubbles introduced into the fluidor particles of solid matter entrained with the fluid. Even slightdisturbances of the homogeneity in the moving fluid may lead toconsiderable errors in the measured viscosity value X_(η) which are ofthe order of up to one hundred times the actual viscosity, η, of thefluid.

By evaluating a number of waveforms of the excitation current i_(exc)which were recorded during measurements performed in different liquidsthat were disturbed in a predetermined manner, the inventors found totheir surprise that, on the one hand, the excitation current i_(exc) mayvary considerably over time despite essentially unchanged conditions,e.g., in the case of a steadily flowing liquid of constant density andviscosity and with an essentially constant content of entrained airbubbles. On the other hand, however, it was ascertained that theexcitation current i_(exc), which varies in a virtuallyunpredeterminable manner, particularly its amplitude, and thus theintermediate value X₁, exhibits an empiric standard deviation s_(iexc)or an empiric variance which is very closely correlated with the degreeof inhomogeneity.

According to the invention, meter electronics 50 derive from this asecond internal intermediate value X₂, which, serving to assess theinfluence of inhomogeneities in the fluid, which was not taken intoaccount in the formation of the intermediate value X₁, is used in thedetermination of the viscosity value X_(η) to weight the intermediatevalue X₁.

The use of the intermediate value X₂ is based on recognition that, onthe one hand, the intermediate value X₁ alone can provide sufficientlyaccurate information about the viscosity, η, of the fluid only if thefluid is largely homogeneous, and that, on the other hand, as describedabove, the instantaneous inhomogeneities in the fluid can be assessedvery accurately and largely independently of the fluid based on thewaveform of the injected excitation current i_(exc).

In a further preferred embodiment of the invention, to obtain theviscosity value X_(η), the intermediate value X₁ is normalized to theintermediate value X₂ by a simple numerical division, so that theviscosity value X_(η) is

$\begin{matrix}{X_{\eta} = {\frac{K_{1}}{K_{\rho}} \cdot \left( \frac{X_{1}}{X_{2}} \right)^{2}}} & (2)\end{matrix}$

where

-   -   K₁=a device constant, dependent in particular on the geometry of        flow tube 13.

The density value X_(ρ) in the denominator of Eq. (2) only takes accountof the fact that actually the square of the current provides theinformation about the product of density and viscosity, see also U.S.Pat. No. 4,524,610.

Furthermore, it turned out to the inventors' surprise, that indetermining the viscosity value according to Eq. (2), the internalintermediate value X₂ can be easily determined according to the linearrelationship

X ₂ =K ₂ ·s _(iexc) +K ₃  (3)

where

-   -   K₂, K₃=constants determined by calibration which, as is readily        apparent, correspond to the slope and offset, respectively, of a        simple equation of a straight line, cf. FIG. 6.

To determine the two constants K₂, K₃, during a calibration for twocalibration fluids of known and, if possible, constant viscosity and ofinhomogeneities which differ but remain unchanged, both theinstantaneous standard deviation is estimated for the respectiveexcitation current, particularly for its amplitudes, and a ratio X_(η)/ηof the respective measured viscosity value to the respectiveinstantaneous viscosity is formed. The first calibration fluid(subscript I) may be flowing water with air bubbles introduced therein,for example, and for the second calibration fluid (subscript II), watermay be used which is as homogeneous as possible.

For the above-described case where the viscosity is to be determinedaccording to Eq. (2), the two constants K₂, K₃ can be calculated from

$\begin{matrix}{{K_{2} = \frac{\sqrt{\frac{X_{\eta,I}}{\eta_{I}}} - \sqrt{\frac{X_{\eta,{II}}}{\eta_{II}}}}{s_{{iexc},I} - s_{{iexc},{II}}}}{K_{3} = {\sqrt{\frac{X_{\eta,I}}{\eta_{I}}} - {K_{2} \cdot s_{{iexc},I}}}}} & (4)\end{matrix}$

The respective empiric standard deviation s_(iexc) is preferablycalculated by means of a sampling AF of intermediate values X₁, storedin digital form in volatile data memory RAM, for example, according tothe known function

$\begin{matrix}{s_{iexc} = \sqrt{\left( {\frac{1}{m - 1}{\sum\limits_{j = 1}^{m}\left( {X_{1,j} - {\frac{1}{m}{\sum\limits_{j = 1}^{m}X_{1,j}}}} \right)^{2}}} \right)}} & (5)\end{matrix}$

If necessary, the sampling AF serving to determine the standarddeviation may also be a correspondingly stored sampling sequence of anamplitude characteristic of the excitation current i_(exc), i.e., asection of a digitized envelope of the excitation current i_(exc).

Investigations have shown that for a sufficiently accurate estimate ofthe standard deviation s_(iexc), samplings of relatively small size m,e.g., approximately 100 to 1000 intermediate values X₁, are necessary,with the individual intermediate values X₁ having to be sampled onlywithin a very narrow window of about 1 to 2 seconds. Accordingly, arelatively low sampling frequency on the order of few kilohertz, e.g.,about 1 to 5 kHz, would be sufficient.

The intermediate value X₂ can advantageously also be used to signal thedegree of inhomogeneity of the fluid, or measured values derivedtherefrom, such as a percentage of air contained in the fluid or acontent by volume or mass of particles of solid matter entrained withthe fluid, e.g., on site or in a remote control room in a visuallyperceptible manner.

The viscosity value X_(η) determined according to Eq. (2) represents agood estimate of a dynamic viscosity of the fluid, which, as is wellknown, may also be obtained as the product of the kinematic viscosityand the density ρ of the fluid. If the viscosity value X_(η) is to serveas an estimate of the kinematic viscosity, a suitable normalization tothe density value X_(ρ) must be performed prior to its output, e.g., bya simple numerical division. To that end, Eq. (2) may be modified asfollows:

$\begin{matrix}{X_{\eta} = {K_{1} \cdot \left( \frac{X_{1}}{X_{\rho} \cdot X_{2}} \right)^{2}}} & (6)\end{matrix}$

It also turned out that for such viscometers with such a flexural modetransducer, particularly if the vibration amplitude is maintained at aconstant value, a ratio i_(exc)/θ of the excitation current I_(exc) to avelocity θ of a motion causing the internal frictions and, thus, thefriction forces in the fluid, which velocity is not measurable directly,is a more accurate estimate of the above-mentioned damping thatcounteracts the deflections of flow tube 13. Therefore, in order tofurther increase the accuracy of the viscosity value X_(η), particularlyto reduce its sensitivity to varying vibration amplitudes of flow tube13, in another preferred embodiment of the invention, the intermediatevalue X₁ is first normalized to a velocity value X_(θ), which representsthe above-mentioned velocity θ. Put another way, a normalizedintermediate value X₁* is formed according to the following rule:

$\begin{matrix}{X_{1}^{*} = \frac{X_{1}}{X_{\theta}}} & (7)\end{matrix}$

Based on recognition that, particularly if a flexural mode transducerassembly is used for transducer 10, the motion causing the internalfriction in the fluid very closely correlates with the motion of thevibrating flow tube 13, which is detected by means of sensor 17 or bymeans of sensors 17 and 18, the velocity value X_(θ) is preferablyderived by means of meter electronics 50, e.g., by means of an internalamplitude-measuring circuit 56, from the at least one sensor signal s₁,which has already been digitized if necessary. The use of the at leastone sensor signal s₁ not only has the advantage that, as mentionedabove, practically no basic changes are necessary in the mechanicaldesign of the transducer assemblies of conventional Coriolis massflowmeters, but that it is also possible to use the respective sensorarrangements of such transducer assemblies virtually unchanged.

Using the normalized intermediate value X₁*, the viscosity value maythen be determined from

$\begin{matrix}{X_{\eta} = {\frac{K_{1}}{K_{f} \cdot X_{\rho}} \cdot \left( \frac{X_{1}^{*}}{X_{2}} \right)^{2}}} & (8)\end{matrix}$

The correction factor K_(f) introduced in Eq. (8) serves to weight thedensity value X_(ρ) with the instantaneous vibration frequency of thevibrating flow tube 13.

At this point it should be pointed out once again that the sensor signals₁ is preferably proportional to a velocity of a, particularly lateral,deflection motion of the vibrating flow tube 13; the sensor signal s₁may also be proportional to an acceleration acting on the vibrating flowtube 13 or to a displacement of the vibrating flow tube 13. If thesensor signal s₁ is proportional to a velocity in the above sense, thecorrection factor K_(f) will correspond to the vibration frequency ofthe vibrating flow tube 13, while in the case of a sensor signal S₁proportional to a displacement, the correction factor K_(f) will beequal to the cube of the vibration frequency.

The aforementioned functions serving to generate the viscosity valueX_(η), which are symbolized by Eqs. (1) to (8), are implemented at leastin part in an evaluation stage 54 of meter electronics 50, which isadvantageously realized by means of signal processor DSP, as shown, orby means of microcomputer 55, for example.

The creation and implementation of suitable algorithms correspondingwith the above-described functions or simulating the operation ofamplitude control circuit 51 or frequency control circuit 52, and theirtranslation into program codes executable in such signal processors, isfamiliar to those skilled in the art and, therefore, does not requiredetailed explanation. Of course, the aforementioned equations may alsobe represented, in whole or in part, by means of suitable analog and/ordigital discrete computing circuits in meter electronics 50.

The viscometer according to the invention has an added advantage in thatthe viscosity value provided by it, X_(η), because of its insensitivityto inhomogeneities in the fluid, also exhibits low cross sensitivity tochanges in mass flow rate or density.

1. A method of determining the viscosity of a fluid flowing in a pipe,comprising the steps of: feeding an excitation current into anexcitation assembly mechanically coupled to a flow tube conducting thefluid and vibrating said flow tube for producing internal frictionforces in the fluid; deriving from said excitation current a firstinternal intermediate value, representing friction forces acting in thefluid; producing a sampling of first internal intermediate values; usingthe sampling to determine a second internal intermediate value,representing inhomogeneities in the fluid; and generating a measuredviscosity value representing the viscosity of the fluid by means of saidfirst and said second internal intermediate values.
 2. The method as setforth in claim 1, further comprising a step of: sensing vibrations ofthe flow tube and generating at least one sensor signal representingsaid vibrations.
 3. The method as set forth in claim 2, furthercomprising a step of: deriving from said at least one sensor signal avelocity value representing a velocity of a motion causing the frictionforces acting in the fluid.
 4. The method as set forth in claim 3,wherein: said step of generating said measured viscosity value comprisesa step of using velocity value for normalizing the first internalintermediate value.
 5. A method for generating a viscosity value bymeans of a metering device, said viscosity value representing aviscosity of a fluid flowing in a pipe, said metering device comprisinga vibratory transducer and a meter electronics electrically connected tosaid transducer, said transducer including at least one flow tubecommunicating with the pipe for conducting the fluid, and an excitationassembly mechanically coupled to the at least one flow tube, and saidmeter electronics including a driver circuit electrically connected tosaid excitation assembly, said method comprising steps of: generating anoscillating excitation current by means of said driver circuit andfeeding said excitation current into the excitation assembly; vibratingsaid at least one flow tube conducting said fluid; generating digitalfirst intermediates values, each of said digital first intermediatesvalues representing friction forces within the fluid; and storing saiddigital first intermediate values in a data memory of said meterelectronics to produce a sampling of digital first intermediate values;and determining the viscosity value from said sampling of digital firstintermediate values.
 6. The method as claimed in claim 5, furthercomprising a step of: sensing vibrations of said at least one flow tubefor generating at least one sensor signal representing vibrations ofsaid flow tube.
 7. The method as claimed in claim 6, wherein: said stepof determining the viscosity value comprises a step of determining fromsaid at least one sensor signal a velocity value which represent motionswithin causing friction forces acting in the fluid.
 8. The method asclaimed in claim 6, further comprising a step of: using said at leastone sensor signal for controlling said driver circuit.
 9. The method asclaimed in claim 5, wherein: said step of vibrating said at least oneflow tube conducting said fluid comprises a step of feeding saidoscillating excitation current into an excitation coil of saidexcitation assembly for producing a magnetic field interacting with aarmature fixed to said at least one flow tube.
 10. The method as claimedin claim 5, wherein: said step of generating said oscillating excitationcurrent comprises steps of: generating an amplitude control signalrepresenting an amplitude of said oscillating excitation current forcontrolling said driver circuit of the meter electronics, and generatingan frequency control signal representing a frequency of said oscillatingexcitation current for controlling said driver circuit of the meterelectronics.
 11. The method as claimed in claim 10, wherein: said stepof generating digital first intermediate values comprises at least oneof the steps of determining said first intermediate values from saidamplitude control signal, and determining said first intermediate valuesfrom said frequency control signal.
 12. The method as claimed in claim10, wherein: said step of generating said oscillating excitation currentcomprises a step of using a numerically controlled digital oscillatorand using said amplitude control signal and said frequency controlsignal for controlling said numerically controlled digital oscillator.13. A digital flowmeter, comprising: a vibratable conduit with a mixtureof a liquid and a gas flowing therethrough; a driver connected to theconduit and operable to impart motion to the conduit; a sensor connectedto the conduit and operable to sense the motion of the conduit; and adigital transmitter connected to the conduit, said digital transmitterhaving a void fraction determination system configured to determine agas void fraction of the mixture, and a viscosity determination systemconfigured to determine a viscosity of the liquid in the mixture; and aflow parameter correction system operable to determine a flow parameterassociated with the flowing mixture, based on the gas void fraction andthe viscosity.
 14. The digital flowmeter of claim 13, wherein: saidviscosity determination system comprises an in-line viscometer.
 15. Thedigital flowmeter of claim 13, wherein: said viscosity determinationsystem is operable to determine a viscosity correction factor for use bysaid flow parameter correction system in determining the flow parameter.16. The digital flowmeter of claim 13, wherein: said digital transmittercomprises a self-contained modular unit.
 17. The digital flowmeter ofclaim 13, wherein: said digital transmitter is operable to communicatewith external devices and systems.
 18. The digital flowmeter of claim17, wherein: said digital transmitter is operable to communicate with acentral control system.
 19. A digital transmitter, comprising: atransceiver configured to send signals to, and receive signals from,sensors monitoring a vibrating flowtube and a liquid-gas mixture flowingtherein; an apparent flow parameter determination system to generateapparent flow parameter values of the mixture from the signals; and aflow parameter correction system operable to correct the apparent flowparameter values, based on a viscosity of the liquid within theliquid-gas mixture.
 20. The transmitter of claim 19, further comprising:a viscosity determination system that is operable to determine theviscosity, and further operable to determine a viscosity correctionfactor based on the viscosity for use by the flow parameter correctionsystem.
 21. The transmitter of claim 19 further comprising: a voidfraction determination system that is operable to determine a voidfraction of the gas within the liquid-gas flow, wherein: said flowparameter correction system is operable to correct the apparent flowparameter values, based on the void fraction.
 22. A method, comprisingthe steps of: determining an apparent flow parameter of a gas-liquidmixture flowing through a vibrating flowtube; determining a viscosity ofthe liquid; determining a viscosity correction factor; and determiningan error in the apparent flow parameter, based upon the viscositycorrection factor; and correcting the error in the apparent flowparameter.
 23. The method of claim 22, wherein: determining the apparentflow parameter comprises determining an apparent density or massflowrate of the mixture by observing a deflection of the vibratingflowtube.
 24. The method of claim 23, wherein: determining the viscosityof the liquid comprises exposing an in-line viscometer to the liquid.25. The method of claim 23, wherein: determining the viscosity of theliquid comprises providing a sample of the liquid to a viscometer. 26.The method of claim 23, wherein: determining a viscosity correctionfactor comprises calculating a correction factor using a bubble modelthat assumes that the gas within the liquid-gas mixture is contained asbubbled within the mixture.